4 mm long. Dark brown-black, males are darker than females. The upper surface is covered with golden hairs and the femora are dark.  Species level identification of Psallus based on external features is difficult and frequently impossible, especially for females. Noting the host plant may narrow down the range of possibilities.

Other Psallus spp., particularly Psallus wagneri.  

This species cannot be separated from Psallus wagneri (which also occurs on oak) without dissection of the male genitalia. The male aedaegus has a short, broad sub-apical process (c.f. the longer, broadly curved sub-apical process of P. wagneri). 

On Oak. 

Spring-Summer

common (British Heteroptera checklist (2021) - https://www.britishbugs.org.uk/systematic_het.html 

Rarely recorded in Leicestershire and Rutland.

https://www.britishbugs.org.uk/heteroptera/Miridae/psallus_perrisi.html